1) Field of the Invention
This invention relates to a disc for recording information signals, such as an optical disc or a magneto-optical disc. More particularly, it relates to a disc for recording information signals which is adapted to be chucked to a disc rotating driving device by taking advantage of a force of magnetic attraction.
2) Description of the Related Art
There has hitherto been proposed a disc for recording information signals for recording desired information signals, such as a write-once type optical disc or a magneto-optical disc.
This type of the disc which is of an extremely small diameter and permits of high density recording of information signals, is proposed. For example, a magneto-optical disc which has a diameter as small as 64 mm or less, is proposed.
The magneto-optical disc which is of a small diameter and permits of high density recording is loaded on a disc rotating and driving device and rotationally driven at a higher velocity. Whilst the disc is rotationally driven at an elevated velocity, a light beam is radiated from an optical disc onto fine recording tracks provided in a signal recording layer on a surface of the magneto-optical disc and an external magnetic field is impressed by a magnetic head onto the other surface of the magneto-optical disc for recording desired information signals.
For radiating the light beam on the fine recording tracks, the magneto-optical disc, rotationally driven at an elevated velocity, needs to be retained integrally with a disc table of the disc rotating and driving device as well as to be loaded with its center of rotation aligned high accurately with the axis of the disc table.
For accurately positioning and loading the magneto-optical disc on the disc table, a disc loading system is proposed in, for example, U.S. Pat. Nos. 4,926,410, 4,829,510 and 4,787,009, in which a metallic plate, provided on the magneto-optical disc, is attracted by a magnet provided on the disc table for chucking the magneto-optical disc on the disc table.
Referring to FIG. 1, a magneto-optical disc 100 employed in the disc chucking system taking advantage of the force of attraction by the magnet includes a disc substrate 101 molded from a transparent synthetic material, such as polycarbonate resin, into the shape of a disk. A recording layer for information signals is deposited by vapor deposition, sputtering etc. on a major surface 101a of the disc substrate 101. The other major surface 101b of the disc substrate 101, opposite to the major surface 101a on which the recording layer is deposited, is a write/read surface for information signals, and a light beam is radiated from the major surface 101b on the signal recording layer for recording/reproducing information signals.
The disc substrate 101 is formed with a center opening 102, as shown in FIG. 1, in which is engaged a centering member provided on the disc rotating and driving device. A ferrous metallic plate 103 is provided at the center of the major surface 101a of the disc substrate 101 for closing the center opening 102. The metallic plate 103 is mounted in a recess provided in the major surface 101a of the disc substrate 101 for encircling the center opening 102, and is secured therein with the aid of a double-sided adhesive tape or an adhesive 105.
With the magneto-optical disc 100 in which the metal plate 103 is mounted on the disc substrate 101 with the aid of the adhesive 105, there is a risk that the metal plate 103 be detached from the disc substrate 101 on repeated loading and unloading operations on or from the disc table of the disc rotating and driving device. That is, the force of magnetic attraction between the metal plate 103 and the magnet of the disc table at the time of loading and unloading with respect to the disc table acts as a load and is applied to the bonding area of the metal plate 103 to the disc substrate 101 to deteriorate the adhesive power of the adhesive 105.
For overcoming the inconvenience derived from the mounting of the metal plate with the aid of the adhesive, the present Assignee has proposed a magneto-optical disc in which the metal plate is mounted on the disc substrate without employing the adhesive.
Referring to FIG. 2, a magneto-optical disc 200 includes a disc substrate 201 formed of a transparent synthetic resin, such as a polycarbonate resin, and a recess 204 in one major surface 201a of the disc substrate 201 on which a recording layer for recording information signals is deposited. The recess 204 is formed around a center hole 202 and is adapted for receiving the metal plate 203 therein. A plurality of caulking projections 205 are formed at equiangular intervals on the circumference of the recess 204 for caulking the metal plate 203.
After the metal plate 203 is housed within the recess 204, heat or ultrasonic waves are impressed on the projections 205 for caulking these projections 205 towards the recess 204 as shown in FIG. 3 for supporting the metal plate 203 within the recess 204 by distal ends 205a of these caulked projections 205 for mounting the metal plate on the disc substrate 201.
With the magneto-optical disc 200 in which the metal plate 203 is supported and attached by the distal ends 205a of the projections 205 formed as one with the disc substrate 201, the metal plate 203 is fixedly supported by the disc substrate 201 so that the metal plate 203 remains attached to the disc substrate 201 even after repeated loading and unloading operations with respect to the disc table of the disc rotating and driving device.
However, for forming the disc substrate 201 with projections 205 for supporting the metal plate 203, a metal mold used for molding the disc substrate 201 becomes complex in structure to render the preparation of the metal mold difficult.
Besides, if the projections 205 are formed intermittently on the circumference of the recess 204, the molten synthetic resin becomes non-uniform in flow in those areas of the mold cavity which later form the projections 205. Besides, the molten synthetic resin tends to be cured with time lag to produce a so-called weld line to render it impossible to produce the uniformly cured disc substrate 201. Unless cured uniformly, the disc substrate 201 is deteriorated in optical properties due to residual inner stresses in the disc substrate 201. Above all, these residual inner stresses tend to produce double refraction in the disc substrate 201 to deteriorate information signal recording/reproducing characteristics when the disc substrate is used as a component of the magneto-optical disc 200.
For overcoming the inconveniences derived from intermittent projections for supporting the metal plate, it may be contemplated to provide a continuous annular projection on the circumference of the recess for accommodating the metal plate. However, this solution can hardly be realized because of difficulties met in uniformly caulking the continuous annular projection.
Although the continuous annular projection may be caulked only partially, the portion of the projection which has not been caulked remains in the projected state after having the metal plate supported with respect to the disc substrate. The result is that the disc substrate is increased in thickness by the residual portion of the projection so that the magneto-optical disc prepared using this disc substrate is also increased in thickness. On the other hand, difficulties are raised in designing the disc cartridge accommodating the magneto-optical disc because the disc cartridge needs to be of a constant thickness in order to be used interchangeably.